Present day VCRs employ two closed loop servo systems, one to control the operation of the rotating video heads and the other to control the speed that the video tape moves past the heads. One of these servos is called the capstan servo, and the other is the called the drum servo. However, in addition to these two servo loops, there is a third servo loop which results from the interaction of the capstan and drum servo loops because of the common usage of the same video tape which carries the recorded information.
The capstan servo controls the speed at which the video tape travels through the machine. The speed and position of the video tape must be precisely controlled in order to ensure that the video tape is in the correct position relative to the video heads in the rotating video drum. Moreover, the video tape must move past the audio heads at the prescribed speed. To this end, a capstan reference signal is recorded linearly on the tape for use in identifying the exact position of the video tape relative to the video heads. In order to control the speed at which the video tape is being pulled through the machine, the capstan reference signal is compared to an internal reference signal. If necessary, the speed of the capstan is adjusted, based on a comparison of these two reference signals, to place the recorded signals on the video tape at the proper position to be sensed by the video and audio heads.
The drum servo controls the rotational position of the video heads. In order to control the drum servo a drum reference signal is generated by the drum, and this signal is compared to the same internal reference signal used by the capstan servo to determine the speed and position of the video heads in the drum. If these two reference signals are out of sync, the drum servo corrects the speed and position of the drum to properly position the heads relative to the tape.
If the capstan servo is not operating correctly, the video tape will travel too fast or too slowly past the audio heads, and the audio will have a higher or lower pitch than that which was linearly recorded on the tape. Moreover, since the video tape will be moving past the drum at an incorrect speed, the video heads will be unable to track along on the recorded tracks resulting in a loss of video. Major speed deviations will cause noticeable changes in the linear audio frequency and result in noticeably high repetition rates of noise in the video picture. Minor speed variations may not produce noticeable changes in the audio but will produce visible noise in the picture.
An improperly operating drum servo will cause video problems, but will not adversely affect the linearly recorded audio signal. For example, severe errors in the speed of the video drum will cause the horizontal sync pulses to occur too fast or too slow and result in the picture being tipped to one side or the other. Minor speed or phase variations of the drum from the norm will produce periods of noise in the picture, and this noise appears identical to that caused by a minor variation in the speed of the capstan. As a consequence, this symptom can result in a misdiagnosis of the problem and an attempt to repair or replace the wrong servo.
The failure of other non-related components in the VCR can also cause symptoms that may be misdiagnosed as a servo problem. For example, misalignment of the tape path as well as defects in the video signal processing circuits can cause video noise, noise bars, or other symptoms that sometime look the same as the results of a defective servo loop. A major problem in servicing VCRs is, therefore, that of identifying the section of the VCR that is defective.
Since servo problems are not always confined to the electrical portions of the servos, mechanical defects such as dry or damaged motor bearings, slipping belts or idlers, or residue buildup along the tape path can result in a whole different set of symptoms such as an excessive variation in the audio and video signals.
At the present time servo testing relies on observing the video and listening to the audio in an effort to detect any errors which may occur therein. In addition, other instruments such as oscilloscopes are used to observe the capstan and drum reference signals and other signals relating to the specific servo technology used by the VCR under test. The internal reference signal is not normally available to the service technician and cannot, therefore, be used in troubleshooting. For the reasons explained above, misdiagnosis of servo problems and the needless waste of time and replacement of good components has been a major concern in the servicing of VCRS.